A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
There has been a trend to use increasingly shorter wavelengths in the beam of radiation to increase resolution. This has led to the use of reflective optical elements (mirrors) and an open beam path from the reticle to the substrate, instead of refractive optical elements (glass lenses) and a beam path that passes through windows between different compartments of the lithographic apparatus. At (E)UV wavelengths, effective projection of the pattern would otherwise be impossible, and of course even at longer wavelengths, the omission of obstructions, such as windows, may improve the imaging effectiveness.
However, the omission of windows between different compartments of the lithographic apparatus may have the undesirable side effect that contaminants, in particular contaminant particles that emanate from the substrate during illumination, such as molecules, may travel from the substrate to the optical elements and the reticle or mask, which shortens the useful lifetime of the optical elements and the reticle or mask.
One solution that has been used to reduce this problem is the introduction of a gas lock in the path of the beam between the substrate and the optical elements. According to this technique, the apparatus contains compartments that hold the optical elements and the substrate, respectively. The beam passes through a connection between these compartments. Most of the apparatus is evacuated to a low gas pressure, but gas is intentionally introduced into the connection. The gas traps contaminants that emanate from the substrate. Gas that reaches the optics compartment is pumped out, so that contaminants in the optic compartment may be pumped out with the gas.
The gas lock solution may be successful in preventing contaminants from follow a free trajectory from the substrate to the optical elements. However, it has been found that the flow of gas from the gas lock through the optics compartment to a pump may lead to new problems at the reticle or mask. Particles that are generated by mechanical motions in the optics compartment may be accelerated by the gas flow towards the patterned surface of the reticle or mask. Due to the low pressure in the optics compartment, the gas flow may not prevent these particles from hitting the patterned surface. This may degrade the patterned surface.